We've got about 100,000 years of history to get through, so buckle up and let's get moving.
In the Halo universe in the 26th Century, humans couldn't decide on just one alien race to fight a war against; instead, we went and pissed off a WHOLE BUNCH of alien races who formed a group called The Covenant to wipe us out.
The Covenant are NOT your friends.
Luckily, all of the nations of the world had already united (don't ask me how) to form the United Nations Space Command, or UNSC, a military force trained to fight Earth's battles against any and all pissed-off individuals, human or alien.
The UNSC were all Covenant-stomping badasses, and they were doing their best to stop the Covenant's whole "let's get rid of the humans" agenda, but the war wasn't going super well. I mean, let's be honest: We were kinda getting our asses handed to us.
Even though they fought bravely, the war was not going well for the UNSC.
The most ruthlessly effective Covenant-stomping badasses were called **the Spartans. These cybernetically enhanced soldiers were named after those buff, shirtless Greek dudes who could fight wicked good.
Spartans were the BAMFs of the UNSC. They did not even kind of mess around.
One Spartan in particular, Spartan 117, was the badassest, Covenant-stompiest soldier of all. Even though his given name was John, he went by the nickname "Master Chief."
The man, the myth, the Master.
"What does any of this have to do with a halo?" you're asking. We're getting to that. But first, a UNSC ship in another part of the galaxy, The Spirit of Fire, accidentally stumbled across a planet that the Covenant were VERY interested in.
Anything the Covenant were interested in was decidedly dangerous for the UNSC, so the Spirit of Fire's Captain James Cutter decided to take a closer look.
Captain James Cutter knows that if the Covenant wants it, it's probably bad.
Cutter realized that the Covenant were trying to activate ancient alien weapons technology that would allow them to win the war quickly by totally decimating humans. The crazy-advanced ancient alien race that developed this technology was called The Forerunners.
Covenant control of Forerunner tech = No bueno for humans.
Sgt. Cutter and his crew managed to stop the Covenant plot and blow up the Forerunner planet (!), but their ship's Faster Than Light (FTL) drive was crippled in the process, leaving them trapped in space — which is like your car breaking down on the highway times about a million.
Halo Rule #1: When in doubt, blow s*** up.
Meanwhile, Master Chief was aboard another UNSC ship,
The Pillar of Autumn, that was fleeing from a losing battle when it was forced to crash-land on a different, equally weird planet in another part of the galaxy. This one is shaped like a perfect ring. You might even call it...a halo. *Dramatic music plays!*
If you like it (a galaxy) then you should put a ring (Halo) on it.
Comin' in hot
Never one to waste an opportunity, Master Chief explored the Halo to rescue the crew of his crashed ship. While he was exploring, he discovered that the Halo is also a planet designed by the Forerunners, meaning that the Covenant wanted to control this planet, too. They're really territorial about anything, Forerunners. It's kind of their main thing.
Even though he was outnumbered and outgunned by a hell of a lot, Master Chief managed to beat the Covenant to the Halo's control center with the help of his AI counterpart, Cortana. Think of Cortana as the helpful voice in the Chief's head.
It's Cortana who figured out that the Halo wasn't just a Forerunner weapon, like the ones discovered by The Spirit of Fire. The Halo was the Forerunner weapon, capable of wiping out all sentient life in the universe. The Covenant, however, believed that the Halo was a religious artifact and that activating the ring would begin a holy event that they call "The Great Journey." This was not correct. It would, in fact, kill everybody.
Cortana discovered that the Halo was a Forerunner-designed galaxy bomb.
Because this situation wasn't complicated enough already, while trying to fire the Halo ring, the Covenant accidentally released a flesh-eating virus-like life form trapped by the Forerunners inside the Halo. These gnarly critters were called, collectively, the Flood, and they feasted on the flesh of human and Covenant alike.
If you thought the Covenant were bad, you're really not gonna like the Flood.
With the Covenant in a panic and the Flood contagion threatening to escape the Halo, Master Chief and Cortana used the time-honored UNSC tactic of blowing s*** up. They escaped the Halo, but not before detonating a nuclear reactor that destroyed the whole installation.
Halo Rule #2: See Halo Rule #1
Bingo bango. So we're done here, right? Um, not quite.
Enter the next chapter in the saga: Halo Wars 2.
The destruction of the sacred Halo ring caused quite the uproar among the members of the Covenant, and divisions began to open up within the confederacy. Each new faction accused the others of heresy. One of these rogue factions was, apparently, a collection of Brute-class Covenant called The Banished.
The goals of The Banished were unknown; what was known was that their leader, Atriox, was one of strongest, most brilliant, and most ferocious warriors in the Covenant's thousand-year history. Whatever his aim, he won't be easy to stop.
That's where you come in.
(If you don't believe this history was "abridged," here's a brief list of things we left out: Harvest, the fall of Reach, the Index, 343 Guilty Spark, Rampancy, the Arbiter, the Didact, the Librarian, and the Ark. Or, check out the comments.)
** This post was corrected on February 21, 2017 to clarify that Spartans were, in fact, created to suppress human insurgencies, not developed during the war in response to The Covenant threat.